Dehydration

One thought that I'd had on the subject of hangovers (and I'm not a biologist so this is just a guess) is that it would be caused by a diversion of blood from other organs to the liver in order to help process the alcohol quicker.

The idea for this comes partly from trying to explain to someone why your mouth would become dry in the morning after a night of heavy drinking even though the alcohol would not severely dehydrate you and also the 'fight or flight' response in which blood is diverted from places like the stomach, intestines and bladder to the muscles so you can run faster or fight better. In these cases you may feel sick (from the stomach), having loose bowels (from the intestines) or become incontinent(sp?)(from bladder). The point here being that in the morning if you feel ill it is likely not to be trying to remove alcohol in the stomach but that there is not much blood going to the stomach because you are still trying to process the alcohol in the liver.

Like I said I'm not a biologist and this is just a guess but it seems to make sense to me. I'm sure a more informed person will look over this and tell me how wrong I am.

Blood flow to different areas of the body can be controlled. For instance, it doesn't matter what else happens, blood will keep flowing to the brain, since that's the most important organ in survival. The other two key organs are the heart (not too surprising) and the kidneys.

“Autoregulation” is one of the most important mechanisms in homeostasis. Its function is to maintain the flow of blood to different parts of the body in spite of changes in blood pressure. It does this in various ways.

One example is in the kidney, where a set of cells (called the macula densa) senses changes in the concentration of sodium ions and chloride ions. This is an indication of a change in blood pressure. As a result, the macula densa cells release chemicals known as prostaglandins. These are then detected by cells called granular juxtaglomerular cells which line the arteries supplying blood to the nephrons of the kidneys.

The juxtaglomerular cells then release a chemical called renin, which controls vasocontriction, (narrowing and widening of the arteries).
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It's then just a case of simple physics. If you put your finger over 90% of the outlet aperture of a tap, the water comes spurting out at high speed and pressure. This is because pressure is inversely proportion to area. Hence, if you narrow an artery when there is a lowered flow of blood, you can raise its pressure.

I would have thought that if a vein, artery or capillary if constricted without the heart pumping more forcefully wouldn't cause an increase in pressure as the blood would be more likely to go through an alternate, less constricted, route. i.e. the path of least resistance stuff

I've never understood this on the show. If you get extremely drunk of an evening, the following morning I have an overwhelming desire to drink vast amounts of water, and am demonstrably dehydrated as evidenced by my first trip to the little boys room.

So how can the blood just not get to a certain part of the body under certain conditions that don't involve an artery or a vein being directly blocked?

I'm not sure if I'm reading you correctly (I've just been stood on a freezing hockey pitch for 70 minutes and my brain has gone on strike), but is it that you're implying that you don't understand how the blood completely stops going to a particular part of the body?

In which case, (and I will need to triple check this), the fact is that blood never completely stops going to any part of the body, or else the cells in that part of the body will die from lack of oxygen. It's just that the movement of blood to certain parts of the body is prioritised at times.

In which case, (and I will need to triple check this), the fact is that blood never completely stops going to any part of the body, or else the cells in that part of the body will die from lack of oxygen. It's just that the movement of blood to certain parts of the body is prioritised at times.

That was what I thought - that tissue would die if blood didn't reach it. However when you say that the movement of blood to certain parts of the body is prioritised, it makes me think of blood flow as being something quite chaotic with blood flowing in all sorts of different directions, whereas I had thought of it as following an orderly and predictable route.

It's not like there is one continuous tube flowing around the body - it's a complicated networking involving lots of branching out and later, merging again.

The blood from the heart which travels out through the aorta is almost immediately begins to branch into other main arteries, which carry the blood to the brain, liver, kidneys etc.etc.

The blood continues to branch out into smaller arteries, then arterioles (very small arteries), and then capillaries. Capillaries are the smallest blood vessels, with one-cell-thick walls, so oxygen diffuses out of the blood and into the tissues here. Similarly, CO2 enters the blood stream from the tissues here.

Having branched out, the blood in the capillary starts to merge with the blood from other capillaries into venules (small veins). Then the blood from this venule will merge with blood from others into a vein. The blood is then transported back to the heart again.

Anyway, to actually answer the question given, it is quite organised - the routes are already drawn out, like the roads in, say, the UK. It's just the amount of traffic flowing along those routes which makes the difference.

Let's say London is the heart...

If the amount of traffic on the roads in the country is very large, but you only want 3,000 cars arriving per hour in Southampton, then you reduce the number of lanes on the M3 to two or one.

I think that's the best analogy for it that I've ever heard. It's certainly a well documented fact that blood can be diverted away from certain areas to ensure an adequate supply to others. In the QI episode on differences it states clearly that the blood will stop going to the extremities to ensure the survival of the vital organs (in this case it was the temperature of the blood that was important). In extreme cases (as shown by a programme by professor Robert Anthony Winston) blood can be diverted away from the brain (in the case of an anneurysm (sp?) to prevent further damage from being caused, and in this case you black out). Whether this is the correct explanation for the cause of the feeling of dehydration the morning after I cannot say but certainly the concept of blood being having a somewhat dynamic route around the body is well explained.